1. Technical Field of the Invention
The present invention relates to a gas turbine power generating system that generates electric power and steam, and more particularly to a binary-fluid-cycle gas turbine system wherein steam is injected into the gas turbine.
2. Prior Art
A binary-fluid-cycle gas turbine system known in the prior art, wherein steam is injected into the gas turbine, is disclosed, for example, in the Japanese patent publication No. 34865, 1979 "Dual-action fluid heat engine."
This binary-fluid-cycle gas turbine system (called the Cheng cycle from the name of the inventor) is typically as shown in FIG. 1. The system consists of a choke valve 1, compressor 2, combustion chamber 3, water treatment equipment 4, pump 5, heat exchanger 6, turbines 7 and 8, condenser 9, etc. In the system, air drawn in from the atmosphere is compressed by the compressor 2 and supplied to the combustion chamber 3, and fuel is burned in the compressed air to produce a high-temperature combustion gas, and this combustion gas is used in turbines 7 and 8 thereby driving the compressor 2 and a load. Furthermore, steam is produced in the heat exchanger 6 using the combustion gas output from the turbines, and the condenser 9 removes moisture from the exhaust gas before it is discharged into the atmosphere.
Such a Cheng cycle as described above, has the advantage that the output and thermal efficiency of the turbine can be increased because the flow of combustion gas entering the turbine is larger and the specific heat of the combustion gas is increased because the steam S produced in the heat exchanger 6 is injected into the combustion chamber 3.
The inventor of the present invention developed a binary-fluid-cycle gas turbine system which is an improvement on the aforementioned Cheng cycle, and has applied for a patent in the Japanese patent publication No. 26780, 1996.
The "partial-regeneration binary-fluid gas turbine" disclosed in the Japanese patent publication No. 26780, 1996 is shown schematically in FIG. 2. That is comprised of a gas turbine system provided with a compressor 2 for compressing air, a combustor 3 in which fuel is burned, and a turbine 7 driven by combustion gas and driving the compressor, a mixer 10 using steam S (saturated steam) as the driving source for boosting the pressure of the compressed air and in which the fluids are mixed, a superheater 6 provided downstream of the turbine 7 for heating the mixed gas produced in the mixer 10 using turbine exhaust, an exhaust heat boiler 12 located downstream of the superheater 6 for evaporating water using the turbine exhaust as the heat source, an air line 13 for introducing part of the compressed air produced in the compressor 2 into the combustor 3 and the rest into the mixer 10, a main steam line 14 for transferring part of the steam S produced in the exhaust heat boiler 13 to the mixer, and a mixed gas line 15 for introducing the mixed gas produced in the mixer 10 into the combustor 3 via the superheater 6.
In this partial regeneration binary-fluid gas turbine, steam S is produced by the exhaust heat discharged from the gas turbine, drawn in and mixed with part of the compressed air, and after being heated in the superheater 6 by the exhaust from the gas turbine, the steam is injected into the combustor. Therefore, this type of gas turbine can recover more energy than with a Cheng cycle by the amount corresponding to the temperature increase of the air which is heated by the exhaust from the gas turbine, so the efficiency of the cycle can be increased.
FIGS. 3 and 4 are exhaust heat recovery diagrams for the aforementioned Cheng cycle and the binary-fluid gas turbine. In the figures, the abscissas and ordinates are the heat content of the exhaust gas of the gas turbine during the heat exchange and temperature, respectively. More precisely, the abscissa shows the enthalpy of the exhaust gas from the gas turbine using 0.degree. C. as the reference.
In FIGS. 3 and 4, exhaust gas from the gas turbine is cooled from about 550.degree. C. to about 150.degree. C., and correspondingly, the water is heated up to saturation temperature at which the water is evaporated to produce saturated steam, and then it is further heated to produce superheated steam.
In the Cheng cycle in FIG. 3, after evaporation only the steam is heated to recover heat, however, in the partial regeneration binary-fluid gas turbine system shown in FIG. 4, a gas mixture of steam and air is heated. Therefore, in FIG. 4, the temperature of the gas increases when it is mixed with the compressed air, and also the flow of the mixed gas is increased, so the gradient of the temperature increase is reduced. As a result, an amount of energy corresponding to the hatched area shown in FIG. 4 is effectively utilized more than in the Cheng cycle, so the cycle efficiency is increased correspondingly. Consequently, in this example, the efficiency at the generator end increases from 41.10% to 41.18%.
With the partial-regeneration binary-fluid gas turbine system shown in FIG. 4, the amount of compressed air drawn in the mixer 10 should be increased in order to increase the cycle efficiency. However, if the pressure of the steam produced by the exhaust heat and which drives the mixer is solely increased, the flow of the steam to the mixer 10 is reduced, and as a result, the amount of heat recovered becomes insufficient.
FIG. 5 shows an example of this situation in which the steam pressure is increased from the 20 kg/cm.sup.2 g of FIG. 4 to 63 kg/cm.sup.2 g. Under these conditions, the temperature of the saturated steam is increased from about 210.degree. C. to about 280.degree. C., which means it is closer to the temperature of the exhaust gas. The area between lines for the temperature of the steam and the exhaust gas correspond to the so-called exergy loss (inactive energy), therefore, this is an improvement to some extent, but because the flow of generated steam decreases there is a resulting reduction in the flow of feed water, consequently the heat in the exhaust gas can be recovered only down to about 200.degree. C., so the amount of recovered energy corresponding to the hatched area in FIG. 5 is smaller than that in FIG. 4. Hence, the efficiency at the generator terminals for this example in FIG. 5 is decreased to 40.96% from the 41.18% in the case FIG. 4.